Final answer:
The element with an ionization energy of 738 kJ/mol is magnesium (Mg), not listed in the original question's choices.
Step-by-step explanation:
The element with an ionization energy of 738 kJ/mol is magnesium (Mg). This value is a specific measurement of the energy required to remove the outermost electron from a neutral atom in the gas phase to form a cation. The ionization energy differs from element to element based on factors such as nuclear charge and electron shielding. For the given elements in the options, such as oxygen, sodium, chlorine, and potassium, they all have different ionization energies. We take into account the reported ionization energy of magnesium which is 738 kJ/mol. Comparing this value to the known ionization energies of the listed elements, none of them perfectly match the value given. Thus, the element with an ionization energy of 738 kJ/mol was not among the options provided in the original question. Therefore, we have used our understanding of atomic structure and periodic trends to determine the correct answer based on the contextual clue provided in the reference materials.